(Aug. 18) -- Frozen iceball or hidden ocean? NASA's Cassini spacecraft has renewed debate among planetary scientists over Saturn's geyser-spewing mystery moon, Enceladus.

Cassini mission controllers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., have begun unveiling images of a 40,000-mile-per-hour pass over Enceladus last week. The fly-by, kicking off a two-year extension of the $3.3 billion spacecraft's mission to explore Saturn, passed about 29 miles above the moon's south pole.

"If there is one set of images from this mission that illustrates how skilled we have become as planetary explorers, this is it," says Cassini imaging team leader Carolyn Porco of the Space Science Institute in Boulder, Colo.

Spinning the spacecraft around as it passed overhead to shoot the images, Cassini captured the "tiger stripe" fissures on the moon, from which geysers were first spotted in 2005.

The discovery started a debate over whether "icequakes" grind the frozen crust of the moon enough to warm and release gases there or if a lake hides inside Enceladus, feeding the geysers. Liquid water is seen as a crucial ingredient for life, raising scientific interest in the fly-by.

Just finding geysers on Enceladus surprised scientists, who thought that the 310-mile-wide moon was too small to harbor any significant earthquake activity or a core hot enough to power geysers. The fly-by images reveal more clues to the mystery of the tiger stripes:

• They appear to be about 980 feet deep, with steep walls.

• A snowy fallout of smooth ice lines their sides.

• Ice boulders the size of houses also surround them.

Most intriguing, the imaging team reports signs that the geyser vents clearly have moved up and down the tiger stripes, perhaps icing themselves shut after each eruption.

"The report that they see self-sealing and evidence for movement of the vents up and down along the fractures over time is great news," says Susan Kieffer of the University of Illinois in Urbana. Two years ago, Kieffer and colleagues unveiled the "Frigid Faithful" model of Enceladus, which predicted just such behavior for the vents, explaining that they came from fractures of solid ice releasing trapped gases and cold water vapor. "No liquid water is present, and there is little potential for life" in this model, Kieffer concluded in a recent issue of Science.

However, planetary scientist Jeffrey Kargel of the University of Arizona in Tucson says venting of cold water vapor couldn't force boulders out of the vents onto the surface of Enceladus and that some of the features near the fractures resemble flows of frozen water ice. He leans more toward the "Cold Faithful" model of Enceladus, which has a reservoir of liquid water trapped inside the moon supplying the geysers.

NASA plans a 16-mile-high pass over Enceladus in October, which should bring even higher-resolution photos of the tiger stripes. For any future landing on Enceladus, crevices about 980 feet deep "would present a significant hazard to most landers and rovers," says planetary scientist Francis Nimmo of the University of California, Santa Cruz.

"But the smooth material apparently spewed out by the geyser makes a very attractive target for future spacecraft observations," he adds. "It will tell us a lot about what is going on beneath the surface of Enceladus, and hopefully whether or not there is liquid water down there."

A big question is just how often Enceladus spews its geysers, which are thought responsible for Saturn's broad "E" ring. More images may allow scientists to time the frequency and explain the origins of the geysers.